Montgomery
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Montgomery clear sky chart from 2005-11-06 to 2024-11-05.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	11% 6% 2 6% 5% 5% 5% 4 6% 8% 42%	100817
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 44%	98190
Transparency	Day	36% 8% 12% 20% 18% 7%	104688
	Night	35% 7% 11% 19% 20% 9%	102457
Seeing	Day	12% 7% 18% 24% 24% 15%	14641
	Night	13% 5% 20% 33% 25% 5%	24600

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	15% 10% 3 6% 5% 4 4 3 5% 6% 40%	2836
	11th to 20th	Day	10% 7% 2 5% 3 3 3 3 4 7% 53%	2999
	21st to end	Day	21% 9% 2 6% 3 3 3 3 4 5% 40%	3476
February	1st to 10th	Day	15% 9% 3 6% 4 5% 4 4 5% 7% 38%	1900
	11th to 20th	Day	19% 8% 2 5% 4 3 3 3 5% 6% 42%	1848
	21st to end	Day	15% 7% 2 8% 5% 4 4 3 5% 8% 39%	1570
March	1st to 10th	Day	21% 8% 3 7% 5% 4 4 4 5% 6% 34%	3120
	11th to 20th	Day	11% 6% 3 7% 4 4 4 3 6% 8% 43%	3384
	21st to end	Day	14% 6% 2 6% 4 5% 5% 4 5% 7% 43%	3904
April	1st to 10th	Day	11% 5% 2 6% 5% 4 4 4 5% 7% 46%	2280
	11th to 20th	Day	11% 6% 2 6% 4 4 3 4 6% 8% 46%	2254
	21st to end	Day	7% 4 1 4 5% 4 4 4 5% 7% 54%	2280
May	1st to 10th	Day	7% 4 2 6% 5% 5% 5% 4 7% 10% 45%	4017
	11th to 20th	Day	14% 7% 2 6% 4 4 5% 4 6% 9% 39%	4081
	21st to end	Day	7% 5% 2 6% 5% 4 5% 4 7% 9% 47%	4476
June	1st to 10th	Day	7% 6% 3 8% 6% 5% 6% 5% 7% 10% 36%	2470
	11th to 20th	Day	6% 4 1 5% 5% 5% 7% 5% 8% 11% 43%	2470
	21st to end	Day	6% 5% 3 6% 5% 6% 7% 6% 8% 11% 37%	2470
July	1st to 10th	Day	7% 6% 3 8% 7% 7% 8% 7% 10% 12% 26%	4120
	11th to 20th	Day	6% 5% 3 6% 6% 7% 7% 6% 9% 11% 34%	4120
	21st to end	Day	6% 6% 3 9% 8% 8% 8% 7% 9% 13% 24%	4344
August	1st to 10th	Day	4 3 1 5% 4 5% 6% 5% 9% 13% 45%	3785
	11th to 20th	Day	6% 5% 2 5% 5% 5% 5% 5% 6% 8% 48%	3729
	21st to end	Day	6% 4 2 5% 5% 5% 5% 5% 8% 9% 47%	4180
September	1st to 10th	Day	9% 6% 3 8% 5% 5% 6% 4 6% 8% 39%	2196
	11th to 20th	Day	13% 7% 3 6% 5% 4 5% 3 6% 8% 42%	2090
	21st to end	Day	9% 4 1 7% 4 4 5% 3 5% 8% 49%	2090
October	1st to 10th	Day	10% 4 1 6% 4 4 3 4 5% 5% 55%	2090
	11th to 20th	Day	10% 5% 1 4 3 3 3 3 5% 6% 57%	2090
	21st to end	Day	10% 5% 1 4 4 3 3 3 4 6% 56%	2033
November	1st to 10th	Day	10% 6% 3 5% 4 3 3 3 4 4 55%	1708
	11th to 20th	Day	11% 7% 3 5% 3 3 3 3 4 7% 53%	1710
	21st to end	Day	13% 6% 2 6% 3 4 4 3 5% 7% 46%	1710
December	1st to 10th	Day	14% 7% 3 6% 5% 4 5% 2 4 6% 44%	1710
	11th to 20th	Day	16% 9% 3 6% 4 5% 4 3 4 5% 42%	2153
	21st to end	Day	19% 8% 3 7% 4 4 4 3 4 6% 39%	3124
January	1st to 10th	Night	17% 8% 3 6% 4 4 4 4 5% 6% 41%	4109
	11th to 20th	Night	12% 6% 2 6% 3 3 4 3 5% 6% 49%	3959
	21st to end	Night	20% 12% 3 6% 4 3 3 3 3 4 40%	4180
February	1st to 10th	Night	15% 8% 2 7% 5% 4 4 4 4 6% 42%	2280
	11th to 20th	Night	19% 7% 2 5% 3 3 3 3 4 4 46%	2218
	21st to end	Night	15% 7% 3 5% 4 3 4 3 5% 8% 42%	1884
March	1st to 10th	Night	22% 10% 3 6% 5% 4 4 2 4 6% 35%	3733
	11th to 20th	Night	11% 6% 2 5% 4 4 5% 4 7% 8% 45%	3561
	21st to end	Night	12% 6% 3 7% 4 4 4 4 6% 7% 44%	3754
April	1st to 10th	Night	9% 6% 2 5% 5% 5% 4 4 7% 7% 46%	1900
	11th to 20th	Night	12% 4 2 6% 4 3 3 3 7% 8% 47%	1875
	21st to end	Night	6% 5% 2 4 3 4 4 3 6% 8% 54%	1900
May	1st to 10th	Night	8% 4 2 6% 5% 4 4 4 6% 9% 47%	2876
	11th to 20th	Night	16% 7% 2 7% 5% 4 4 3 5% 7% 41%	2812
	21st to end	Night	5% 4 2 4 4 4 5% 5% 7% 12% 47%	3079
June	1st to 10th	Night	6% 6% 3 8% 6% 7% 6% 6% 9% 11% 34%	1710
	11th to 20th	Night	5% 5% 1 5% 5% 4 5% 4 8% 13% 45%	1710
	21st to end	Night	4 5% 2 7% 7% 6% 6% 5% 9% 12% 38%	1710
July	1st to 10th	Night	7% 7% 3 9% 7% 7% 8% 7% 9% 12% 24%	2840
	11th to 20th	Night	4 5% 2 6% 6% 6% 7% 6% 10% 14% 34%	2840
	21st to end	Night	7% 7% 4 9% 7% 7% 7% 6% 8% 12% 26%	3204
August	1st to 10th	Night	3 4 2 6% 5% 4 5% 6% 9% 13% 44%	3144
	11th to 20th	Night	5% 4 2 5% 5% 4 4 4 7% 11% 49%	3100
	21st to end	Night	5% 4 2 5% 5% 5% 4 5% 8% 10% 48%	3476
September	1st to 10th	Night	7% 6% 2 7% 4 7% 6% 5% 8% 9% 39%	1963
	11th to 20th	Night	12% 7% 2 6% 4 4 5% 4 6% 8% 42%	2090
	21st to end	Night	7% 5% 2 6% 5% 4 4 5% 6% 8% 48%	2090
October	1st to 10th	Night	8% 4 2 5% 3 3 4 3 5% 6% 57%	2090
	11th to 20th	Night	10% 3 2 4 3 3 3 2 5% 6% 58%	2090
	21st to end	Night	12% 4 2 5% 3 3 4 2 4 5% 56%	2559
November	1st to 10th	Night	8% 3 2 5% 3 3 4 3 4 5% 57%	2470
	11th to 20th	Night	12% 4 2 4 4 3 3 2 5% 6% 57%	2470
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 6% 50%	2470
December	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 5% 6% 46%	2470
	11th to 20th	Night	16% 8% 2 6% 4 3 3 3 3 5% 47%	3042
	21st to end	Night	17% 7% 3 7% 5% 4 4 3 5% 7% 37%	4532

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	44% 4 5% 19% 23% 6%	2837
	11th to 20th	Day	31% 4 6% 26% 25% 8%	2999
	21st to end	Day	46% 4 6% 17% 18% 9%	3476
February	1st to 10th	Day	45% 4 8% 17% 19% 7%	1900
	11th to 20th	Day	43% 5% 5% 16% 22% 9%	1858
	21st to end	Day	44% 5% 8% 16% 18% 9%	1570
March	1st to 10th	Day	49% 6% 9% 15% 15% 6%	3120
	11th to 20th	Day	37% 6% 8% 19% 20% 9%	3384
	21st to end	Day	41% 6% 11% 17% 18% 8%	3904
April	1st to 10th	Day	36% 6% 10% 21% 17% 9%	2280
	11th to 20th	Day	35% 6% 9% 22% 20% 9%	2254
	21st to end	Day	29% 6% 9% 24% 27% 5%	2280
May	1st to 10th	Day	29% 9% 13% 26% 20% 3	4054
	11th to 20th	Day	40% 8% 13% 25% 13% 2	4085
	21st to end	Day	31% 9% 13% 27% 19% 2	4489
June	1st to 10th	Day	40% 10% 15% 22% 12% 2	2470
	11th to 20th	Day	31% 10% 17% 24% 15% 2	2470
	21st to end	Day	36% 13% 17% 19% 12% 3	2470
July	1st to 10th	Day	41% 17% 24% 12% 4 1	4055
	11th to 20th	Day	37% 15% 20% 22% 5% 1	4051
	21st to end	Day	45% 17% 18% 16% 4	4205
August	1st to 10th	Day	25% 9% 16% 30% 19% 1	3773
	11th to 20th	Day	31% 10% 13% 27% 18% 2	3765
	21st to end	Day	28% 11% 17% 24% 16% 3	4179
September	1st to 10th	Day	41% 9% 14% 15% 16% 5%	2196
	11th to 20th	Day	40% 7% 12% 17% 19% 4	2090
	21st to end	Day	33% 6% 10% 21% 24% 7%	2090
October	1st to 10th	Day	27% 3 8% 12% 27% 24%	3480
	11th to 20th	Day	17% 3 5% 16% 25% 34%	3458
	21st to end	Day	20% 3 6% 17% 40% 13%	3334
November	1st to 10th	Day	33% 4 7% 17% 28% 12%	1708
	11th to 20th	Day	32% 4 7% 17% 27% 13%	1710
	21st to end	Day	38% 5% 9% 17% 22% 9%	1710
December	1st to 10th	Day	41% 5% 10% 17% 20% 6%	1710
	11th to 20th	Day	45% 4 6% 15% 24% 6%	2150
	21st to end	Day	46% 4 6% 22% 19% 3	3124
January	1st to 10th	Night	42% 6% 6% 15% 25% 6%	4109
	11th to 20th	Night	34% 5% 5% 23% 22% 10%	3959
	21st to end	Night	48% 4 4 15% 23% 6%	4180
February	1st to 10th	Night	43% 4 7% 17% 19% 11%	2280
	11th to 20th	Night	41% 5% 6% 14% 22% 12%	2219
	21st to end	Night	39% 7% 9% 16% 16% 13%	1884
March	1st to 10th	Night	50% 5% 9% 15% 13% 8%	3733
	11th to 20th	Night	34% 7% 10% 16% 22% 10%	3561
	21st to end	Night	38% 6% 8% 19% 21% 9%	3754
April	1st to 10th	Night	36% 6% 11% 16% 20% 11%	1900
	11th to 20th	Night	32% 6% 8% 19% 24% 11%	1875
	21st to end	Night	27% 5% 11% 25% 23% 9%	1900
May	1st to 10th	Night	29% 10% 14% 23% 20% 5%	2904
	11th to 20th	Night	41% 7% 13% 23% 13% 3	2813
	21st to end	Night	25% 10% 16% 27% 18% 3	3087
June	1st to 10th	Night	40% 12% 16% 17% 13% 2	1710
	11th to 20th	Night	29% 9% 19% 26% 15% 2	1710
	21st to end	Night	34% 11% 17% 21% 13% 5%	1710
July	1st to 10th	Night	46% 16% 21% 12% 4 1	2795
	11th to 20th	Night	35% 15% 23% 24% 4	2786
	21st to end	Night	48% 13% 19% 15% 5% 1	3102
August	1st to 10th	Night	27% 11% 17% 32% 12% 2	3132
	11th to 20th	Night	27% 9% 18% 27% 15% 3	3130
	21st to end	Night	29% 9% 18% 24% 18% 3	3476
September	1st to 10th	Night	37% 13% 15% 14% 14% 6%	1963
	11th to 20th	Night	38% 9% 12% 19% 16% 7%	2090
	21st to end	Night	32% 7% 11% 19% 21% 9%	2090
October	1st to 10th	Night	29% 4 8% 12% 23% 24%	3480
	11th to 20th	Night	16% 3 4 14% 31% 33%	3461
	21st to end	Night	21% 3 4 18% 34% 19%	4208
November	1st to 10th	Night	28% 5% 8% 17% 26% 16%	2470
	11th to 20th	Night	30% 4 5% 19% 28% 14%	2470
	21st to end	Night	36% 5% 7% 19% 21% 12%	2470
December	1st to 10th	Night	37% 6% 10% 19% 23% 6%	2470
	11th to 20th	Night	41% 4 6% 14% 29% 6%	3044
	21st to end	Night	44% 6% 8% 19% 21% 3	4532

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	15% 19% 30% 27% 7% 1	270
	11th to 20th	Day	17% 15% 29% 23% 13% 3	270
	21st to end	Day	17% 20% 35% 22% 5% 1	297
February	1st to 10th	Day	20% 14% 36% 21% 9%	270
	11th to 20th	Day	27% 15% 32% 21% 5%	270
	21st to end	Day	15% 22% 38% 22% 4	225
March	1st to 10th	Day	20% 14% 31% 26% 9% 1	270
	11th to 20th	Day	14% 11% 33% 29% 12% 1	403
	21st to end	Day	17% 10% 31% 31% 11% 1	506
April	1st to 10th	Day	14% 13% 26% 28% 15% 4	460
	11th to 20th	Day	12% 13% 26% 28% 17% 5%	452
	21st to end	Day	10% 8% 25% 28% 23% 6%	460
May	1st to 10th	Day	8% 6% 21% 29% 27% 8%	463
	11th to 20th	Day	7% 3 17% 28% 28% 16%	487
	21st to end	Day	10% 4 14% 25% 28% 20%	529
June	1st to 10th	Day	9% 3 9% 26% 29% 24%	490
	11th to 20th	Day	8% 1 12% 18% 30% 31%	490
	21st to end	Day	8% 2 9% 23% 27% 32%	490
July	1st to 10th	Day	6% 1 5% 16% 33% 38%	485
	11th to 20th	Day	6% 1 3 15% 30% 44%	484
	21st to end	Day	4 1 5% 14% 38% 37%	532
August	1st to 10th	Day	9% 1 7% 18% 32% 34%	486
	11th to 20th	Day	9% 2 6% 17% 37% 30%	479
	21st to end	Day	12% 2 7% 18% 34% 27%	539
September	1st to 10th	Day	9% 3 7% 22% 33% 25%	490
	11th to 20th	Day	15% 4 7% 21% 37% 16%	490
	21st to end	Day	11% 4 11% 24% 33% 17%	490
October	1st to 10th	Day	11% 3 20% 32% 25% 9%	490
	11th to 20th	Day	10% 7% 16% 29% 29% 8%	490
	21st to end	Day	12% 9% 20% 28% 25% 6%	425
November	1st to 10th	Day	16% 4 20% 35% 18% 8%	282
	11th to 20th	Day	16% 6% 31% 30% 14% 3	270
	21st to end	Day	12% 10% 29% 29% 18% 2	270
December	1st to 10th	Day	20% 10% 34% 21% 13% 2	270
	11th to 20th	Day	23% 13% 27% 28% 8%	270
	21st to end	Day	25% 11% 25% 27% 11%	297
January	1st to 10th	Night	20% 9% 33% 32% 7%	760
	11th to 20th	Night	19% 8% 28% 36% 10%	760
	21st to end	Night	19% 9% 34% 34% 4	836
February	1st to 10th	Night	18% 8% 29% 40% 6%	760
	11th to 20th	Night	22% 7% 33% 32% 6%	743
	21st to end	Night	17% 11% 33% 35% 3	628
March	1st to 10th	Night	14% 9% 31% 37% 8%	760
	11th to 20th	Night	14% 6% 34% 33% 13%	760
	21st to end	Night	14% 8% 30% 36% 12% 1	798
April	1st to 10th	Night	10% 10% 28% 36% 16%	570
	11th to 20th	Night	13% 6% 27% 34% 18% 1	563
	21st to end	Night	9% 4 22% 41% 24% 1	570
May	1st to 10th	Night	8% 4 20% 39% 25% 4	570
	11th to 20th	Night	9% 3 14% 40% 29% 6%	561
	21st to end	Night	8% 2 9% 36% 38% 8%	617
June	1st to 10th	Night	8% 2 8% 33% 42% 6%	570
	11th to 20th	Night	5% 2 8% 23% 50% 12%	570
	21st to end	Night	5% 6% 29% 45% 15%	570
July	1st to 10th	Night	8% 3 21% 53% 14%	555
	11th to 20th	Night	5% 1 2 20% 49% 24%	552
	21st to end	Night	6% 1 4 21% 53% 15%	624
August	1st to 10th	Night	5% 1 3 23% 49% 19%	567
	11th to 20th	Night	8% 1 6% 20% 51% 14%	564
	21st to end	Night	10% 2 7% 24% 43% 14%	627
September	1st to 10th	Night	10% 1 8% 24% 45% 11%	633
	11th to 20th	Night	15% 1 9% 30% 38% 7%	760
	21st to end	Night	8% 2 11% 34% 41% 4	760
October	1st to 10th	Night	9% 2 13% 41% 32% 4	760
	11th to 20th	Night	11% 4 17% 37% 29% 3	760
	21st to end	Night	13% 4 17% 39% 25% 2	836
November	1st to 10th	Night	10% 5% 17% 43% 23% 2	760
	11th to 20th	Night	12% 5% 27% 39% 16% 1	760
	21st to end	Night	14% 6% 25% 39% 14% 1	760
December	1st to 10th	Night	16% 6% 27% 36% 14% 1	760
	11th to 20th	Night	22% 5% 27% 35% 10% 1	760
	21st to end	Night	22% 7% 27% 35% 8%	836

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